This invention generally relates to the field of antennas, more particularly, to an antenna that uses a Nickel-Titanium (Ni--Ti) radiating element.
The explosive growth of cellular radiotelephone systems has resulted in extensive use and handling of mobile phones by subscribers. One of the important considerations in designing a small communication device, such as a cellular phone, is the physical characteristics of its antenna and the interconnecting mechanism used to interface the antenna with radio frequency (RF) circuitry at a termination contact. Typically, it is desirable to design a thin antenna with a termination contact that can withstand day-to-day handling, including occasional mishandling.
In order to survive handling abuse, some conventional antennas use a rigid radiating element that is over-molded with flexible material. The over-mold material is used to limit bending of the rigid radiating element and to evenly distribute the bending stresses between rigid and flexible sections. The use of rigid radiating elements, however, limits the size and flexibility of the antenna. Instead of rigid radiating elements, Ni--Ti radiating elements, which have low electrical resistance and high flexural properties, have been used in antennas. The flexural properties of Ni--Ti radiating elements makes them specifically suitable for cellular phone antennas, which must tolerate dropping and extreme bending without permanent distortion. The flexural properties of Ni--Ti radiating elements are obtained by heat treating to create a specific phase structure. Subsequent mechanical working of the material, such as rolling or forming, creates a suitable material modulus that gives Ni--Ti alloy its properties.
Termination contacts, which provide an electrical interface with RF circuitry, may be positioned at various points along the antenna. For example, termination contacts may be positioned on transmission lines, RF connectors, ground planes, tuning structures, or multiple radiating elements. Conventional antennas employ one of three types of termination contacts: metallic contacts, crimp type compressive contacts, and metal filled conductive polymer contacts. However, for the reasons given below, these contacts are not optimized for Ni--Ti radiating elements.
The Ni--Ti alloy is difficult to join via soldered, brazed, or welded metallic contacts. This is because heating of the Ni--Ti alloy to high temperatures needed for soldering, brazing or welding destroys the mechanically induced temper and may also change the phase structure. Moreover, when sufficiently heated, the surface of the Ti--Ni alloy forms a naturally stable oxide surface of titanium known as RUTILE, which resists wetting by most common solders.
Due to problem with metallic contacts, crimp type compressive contacts are the most common contact system for antennas with Ni--Ti radiating elements. Crimp type compressive contacts are formed by a metallic element suitable for crimping, such as stainless steel or copper beryllium alloy, which is mechanically deformed to create a compressive contact. Under this arrangement, the interface metal must resist deterioration caused by moisture borne environmental contaminants. Typically gold or nickel metallization systems are used to ensure an electrochemically stable contact. However, the cost of corrosion resistant metallization with precious metals is relatively high. Furthermore, the size of the compressive contact is driven by the contact pressure required to insure exclusion of moisture during the life of the antenna, which limits its minimum size.
For metal filled conductive polymers contacts, epoxies filled with conductive metals such as silver or gold are commonly used. However, the epoxy resins are rigid and limit flexibility. Consequently, plasticizer additives are used to increase elastic properties. These additives suffer from changing physical properties over time, generally decreased flexibility from aging or loss of the plasticizer. Because the electrical interconnection is formed by surface contact of the metal particles in the polymer matrix, changing physical properties produce shifts in the electrical conductivity. Moreover, polymer compounds with chemically active metal fillers, such as silver, frequently suffer from electromigration in humid conditions where free electrolytes from the environment are present, causing changes in the contact resistance over time.
Accordingly, a termination contact for an antenna that incorporates a Ni--Ti radiating element is needed that is mechanically and electrically stable over time. The termination contact for such antenna should withstand environmental extremes and mechanical stresses common to hand held cellular phones during its operational life. It is also desired for the contact to have minimum volume to meet miniaturization requirements and be adaptable to high volume automated manufacturing.